A FTIRAS STUDY OF CO ADSORBED AT NI(100) SURFACES

Adsorption of CO at Ni(100) surfaces has been studied with Fourier-Tra nsform Infrared Absorption-Reflection (FTIRA) spectroscopy in the temp erature range 121 K to 350 K. Up to a coverage of THETA = 0.5 top site and bridge site bonded CO has been identified by absorption peaks in the frequency range 2016 to 2048 cm-1 and 1885 to 1949 cm-1, respectiv ely. The site population is temperature dependent, top sites are prefe rred at high temperatures, bridge sites at low temperatures. Towards t he saturation coverage, THETA almost-equal-to 0.67 the top site peak i s replaced by a broad absorption structure between 2000 and 2100 cm-1, which contains at least 3 separate peaks. The bridge site related abs orption remains a single peak but shifts upwards by 20 cm-1 to 1968 cm -1. This suggests that above a coverage of THETA = 0.5 CO molecules ar e arranged in an incommensurate adsorption structure, in which a uniqu e top adsorption site does no longer exist.